1. Field of the Invention
The present invention relates to a power transmission unit arranged between rear wheel axles of, for instance, an FF (Front engine Front drive) part-time 4-wheel drive vehicle.
2. Description of the Prior Art
A prior art power transmission unit of this kind is disclosed in Japanese Patent Publication No. 58-184322. In this prior art power transmission unit, there are provided clutches between an input member for receiving a torque input and first and second output members comprising rotation shafts. In normal driving, the torque input is transmitted from the input member to the output members through the clutches, while, if one wheel goes into muddy soil and the other on a paved road, the driving force of the input member may be transmitted to, for instance, the first output member which corresponds to the wheel on the paved road. In cornering, a clutch corresponding to an outer wheel is automatically disconnected according to the increase of the angular speed of the outer wheel to cause, for instance, the second output member which corresponds to the outer wheel to idle.
In the prior art power transmission unit, however, the input member and the output members are kept connected through the clutches so long as no difference is caused between the angular speeds of the first and second output members. As a result, if the prior art power transmission unit installed between rear wheel axles of, for instance, an FF part-time 4-wheel drive vehicle is operated in a 2-wheel driving mode, the connection between an engine and the input member may be cut by disconnecting a clutch for selecting the 2-wheel drive and 4-wheel drive modes, but extremely large running resistance is applied for the vehicle because the rotation of rear wheels rotate the propeller shaft through the first and second output members and the input member. Accordingly, this kind of power transmission unit requires hub clutches for the rear wheels.